Mentagrade is the measurement of how mental something is.
In this context the word mental is synonymous with words such as 'extreme', 'insane' and 'off the hook'.
The unit of Mentagrade is simply expressed as M. Not to be confused with Moles, in Chemistry.

Something's absolute magnitude in Mentagrade is difficult to calculate, and especially when something particularly mental is happening, it is unlikely you have the right apparatus to hand; so the value of said phenomenon in Mentagrade is usually an intelligent estimate.
Mentagrade is a metric measurement, but the concept of zero is not so simple. 'Room temperature' is considered to be around 2.1M, to two significant figures; this value is calculated by obtaining a weighted average of different locations' mental magnitude; (considering anything particularly mental an anomaly and not including it in the average).
To understand the concept of zero M, we need to consider the idea of Mentagrade being dynamic; so we think of it as change in Mentagrade - or ΔM - In effect, we measure how much the Mentagrade has gone up/down by as a consequence of something happening.
If something extremely boring happens, then the value of ΔM is going to be negative. We need the concept of zero because if we are taking a non-dynamic snapshot of something's mental magnitude, we measure ΔM from zero, so we do not need to express it as ΔM, but rather as just M, as it does not have any concept of time - if we are measuring ΔM at a point, then we measure it from 'room temperature' (2M).

The scale of Mentagrade is continuous and infinite, but something has to be really significantly mental to score above 10M.

Examples of Mentagrade measurements:

Shouting out at least twelve synonyms of the word 'sex' in physics: 7.3M

Kicking a pigeon: 6.2M

In a football game, the losing team's fans from the start of the game to the end: ΔM=6.9M

Planning and discussing on burning Mr Thrower: 7M.

Actually burning Mr Thrower: 26.4M.

Getting half the world's population to change their shoelaces on Valentines day: 86M.

Miles: Woah, did you know that yesterday, en masse, the UK made 436123 tonnes of batter for pancake day?